1. Field of the Invention
This invention relates to electrical elements for limiting current passing through equipment, and more particularly relates to a peak-current-limiting element for protecting electrical equipment. Moreover, an energy savings is thus realized in the use of this invention by controlling energy consumption.
2. Description of the Prior Art
The use of various electrical elements have been used in the past as protectors. One is a solid state current dependent resistor type device. Such a device is a specially formulated metal oxide ceramic material that is capable of surpressing high inrush current surges. Such a device consists of two wires connected in series with a load and possesses the characteristic that the resistance decreases from a high value to low value once current flow is initiated. Such a device is only available in fixed increments of "cold" resistance values, usually of relatively low values. Thus, the degree of control is limited. Further, once the circuit where the device is employed is energized and the device is in full conduction, it will no longer provide the current limiting function. Therefore, if a transient occurs on the system after the limiting device is in full conduction, the device will provide no further current limiting during the surge.
Another type protector is series resistors that are shorted out by means of a contactor or solid state semiconductor device after an initial surge. This protector uses a fixed resistor inserted in series with an electrical load to provide current limiting. Such resistor is shorted out by either a contactor or a solid state switch (semiconductor) after either a predetermined period of time or after the voltage across the load has reached a predetermined value. Such shorting out immediately after the initial energizing of the load allows no further inrush protection in the event of a voltage surge or transient on the power bus. Further, the current-limiting function is only active during the application of voltage to the circuit and is not resettable until the circuit is deenergized and power is re-applied. Moreover, the circuit does not directly sense current and thus provide a continuous control of the peak-current value.
Another type protector is a ramp increase of solid state switch conduction (phase-angle control) for AC use or ramp increase of pulse--width--modulated chopper control for DC use. This protector utilizes variable conduction times of portions of the total frequency to adjust the effective or average voltage to the load to accomplish a current-limiting function. This protector allows little or substantially no limiting of the peak value of the current for DC or square wave voltage sources. This is particularly true of the pulse-width modulated scheme where the duty-cycle of the applied voltage is varied to achieve a low average value of current. In such instance the full amplitude of the source voltage is applied to the load and thus the peak current is not limited. With phase-angle control of a sinusoidol power-source wave, the amplitude of the peak current is somewhat controllable by controling the conduction angle. However, the minimum conduction angle usually achievable represent a minimum peak voltage that is 5 to 10 percent of the source voltage. Therefore, total peak--current--limit control is not achieveable. Further, the circuit necessary to achieve either phase-angle control or pulse--width--modulated control with a current feedback has numerous parts and complex circuitry creating a large package size. Moreover, additional circuitry must be incorporated in each of these methods to provide a time rate of increase of the duty cycle (phase-angle) or a current dependent decrease in duty cycle (phase angle) to allow achieving a current-limit control. Again, this increases the circuit complexity. And, unless the current-dependent decrease in duty cycle (phase angle) is employed, current limit can be achieved only during the initial energizing of the load. Therefore, an this case, no current-limit protection would exist for a voltage transient surge that occurs after the initial energization.
During the active operation of this switching type circuitry, the power source waveform is chopped to achieve the average-current limiting effect. Thus, during this time harmonics in the power system and load exist because of the chopping effects. Such can result in interactions and interferences with certain type loads.